1. Field of the Invention
The present invention relates to an image signal processing apparatus which is capable of accurately outputting a pulse-width modulated signal in response to an image signal and a triangle-shaped wave signal supplied thereto, and capable of substantially increasing the number of density tones by making the pulse width of the pulse-width modulated signal stable and narrow.
2. Description of the Related Art
As a conventional technique in which an image signal and a triangle-shaped wave signal are inputted therein to output a pulse-width modulated signal, an image signal processing apparatus 2 shown in FIG. 1 can be proposed. FIG. 2 shows waveforms subjected to predetermined processing by the image signal processing apparatus 2. The image signal processing apparatus 2 comprises a comparator 6, an integrator 4 for introducing an integrated signal B as a triangle-shaped wave into one of input terminals of the comparator 6 in synchronism with a pixel synchronizing signal A, and an image signal generator 8 for outputting an image signal C therefrom in synchronism with the pixel synchronizing signal A so as to introduce the image signal C into the other of the input terminals of the comparator 6.
The comparator 6 of the image signal processing apparatus 2 constructed as described above serves to output a pulse-width modulated signal D shown in FIG. 2. As is seen from FIG. 2 , the pulse-width modulated signal D has its pulse width proportional to the level of the image signal C.
There is a demand for high speed image signal processing in the field of application of the image signal processing apparatus 2. This demand is intended to enhance the added value, i.e., performance of a system used with the image signal processing apparatus 2 by effecting such high-speed signal processing.
However, when it is desired to process the image signal at high speed in the conventional image signal processing apparatus 2, delays in time with respect to an image signal C such as those shown in FIGS. 3 and 4 appear at the time of a fall and a rise of the image signal C.
As is understood from the same drawings, a problem arises in that pulse widths Pa, Pb, Pc of the pulse-width modulated signal D corresponding to an image signal having the same level III vary according to the levels (I, II, III in the drawings) of the image signal C relative to a pixel immediately before the present pixel.
On the other hand, it is generally necessary to increase the number of density tones corresponding to an area of lightness of an image, that is, to widen the dynamic range of the image density in order to accurately reproduce the density of the image. When a pulse-width modulation system is used, the dynamic range of the image density is determined by the ratio of the maximum pulse width to the minimum pulse width.
It is therefore desirable to set the minimum pulse width as narrow as possible with a view to widening the dynamic range of the image density.
When it is desired to narrow the pulse width of the pulse-width modulated signal D in the conventional image signal processing apparatus 2, it is necessary to make the maximum level of an integrated signal B greater than the maximum level of the image signal C and make smaller the difference (amount of overdrive) between the maximum level of the integrated signal B and the maximum level of the image signal C, as shown in FIG. 5.
However, if the amount of overdrive is made excessively small, the comparator 6 is liable to cause unstable operation, and the pulse width is rendered inconstant. Since there is a limit to the high-speed response characteristics of the integrator 4, a so-called waveform rounding takes place at the tip portion of the integrated signal B as shown in FIG. 6. As a consequence, the amount of narrowing of the minimum pulse width is limited due to the unstable operation of the comparator 6 and the waveform rounding.